The present invention relates to compositions and methods for preparing dispersions.
New printing processes such as ink jet printing have led to a demand for paper having specific properties while being useful for multiple purposes, such as reprographic copying, laser printing, ink jet printing, and the like. Specialty papers have been developed for each type of application, but as a practical matter, a multipurpose paper suitable for all such uses is desirable. In particular, ink jet printing demands that both ink and paper perform in such a way as to provide an acceptable image by wet printing, and acceptably rapid drying of the ink. Additives and agents, including sizing agents, are commonly used to impart to paper some of the properties needed for applications such as ink jet printing.
Paper is made with and/or surface treated with sizing agents primarily to prevent excess penetration, wicking or spread of water or ink. Many different types of nonreactive and reactive sizing agents are well known in the papermaking industry.
Sizing agents for paper are often provided in the form of aqueous dispersions. Such dispersions can contain one or more sizing agents, one or more salts, and one or more processing aids.
During use or during short-term storage, dispersions containing sizing agents and salts can stratify, resulting in an upper layer containing a higher than average concentration of the sizing agent and a lower layer containing a higher than average concentration of salts. This is a significant disadvantage because frequent or continuous agitation can be required in order to maintain a substantially uniform dispersion.
Stratification of liquid rosin sizes has been addressed in U.S. Pat. No. 2,873,203, the disclosures of which are hereby incorporated herein by reference in their entirety. The disclosed method for inhibiting stratification includes the addition to the rosin size of a small amount of sodium chloride, e.g., up to about 5% based on the total weight of solids in the size. However, the data indicate that although stratification can be eliminated for 2 days with the addition of up to 5% sodium chloride, the addition of more sodium chloride can lead to salting out of the sodium chloride rather than extension of the period during which stratification is prevented.
The present invention provides compositions and methods for forming dispersions that can remain substantially uniform during use and/or storage. The compositions and methods are useful in dispersions of materials such as sizing agents for paper.